Optical recording medium, recording material, method of producing optical recording medium, and optical recording, reading and rewriting method

ABSTRACT

An optical recording medium including a recording layer which includes a reversible phase-change recording material, the recording layer being capable of writing information therein, reading written information therefrom, and rewriting written information by utilizing the reversible phase change of the phase-change recording material, wherein when a recording mark formed in the recording layer is repeatedly read 5000 times, using a continuous wave laser beam having such an intensity Pr that satisfies the condition of 1.1≦R≦2.0, in which R is σ repeat /σ 1  as defined in the specification, the optical recording medium satisfies a relationship of d 1 &gt;d ow  as defined in the specification. The phase-change recording material, a method of producing the optical recording medium, and a method of writing information in the optical recording medium, reading written information therefrom and rewriting written information therein are proposed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical recording medium comprisinga recording layer which is capable of writing information therein,reading written information therefrom, and rewriting written informationtherein. The present invention also relates to a recording material foruse in the recording layer of the optical recording medium.

The present invention also relates to a method of producing the opticalrecording medium.

The present invention also relates to a method of writing information inthe optical recording medium, reading written information therefrom, andrewriting written information therein.

2. Discussion of Background

As an optical recording medium capable of writing information therein,reading written information therefrom, and rewriting written informationtherein, with a light beam irradiation, there is well known aphase-change optical recording medium, which utilizes the reversiblephase changes between a crystalline phase and an amorphous phasethereof.

The inventors of the present invention have studied phase-change opticalrecording media, in which an Sb—Te recording material and an Ag—In—Sb—Terecording material are used, and have discovered that the phase-changeoptical recording media using these recording materials have excellentcharacteristics, for example, in terms of C/N, erasing ratio,sensitivity, jittering, preservation stability, repeated writing andreading stability (hereinafter referred to as reading opticalstability).

Attention has been paid to the recent trend that the optical recordingmedia are being developed into DVD media. In comparison withconventional CD media, the DVD media have a larger capacity, so that itis required that the response or correspondence to recording linearvelocity (hereinafter referred to as the correspondence to recordinglinear velocity) be higher and that the beam spot on a drive side besmall.

As the conventionally proposed Sb—Te recording material and Ag—In—Sb—Terecording material, there have been employed materials with such acomposition that can secure the preservation stability and the readingoptical stability.

When attention is paid to Sb and Te of the constituent elements of theabove-mentioned recording materials, the compositions in the shade areain the graph in FIG. 1 are used in the conventional CD media, in whichgraph the composition data are plotted with the recording linearvelocity and the preservation stability (both with an arbitrary unit) asordinate and Sb/(Sb+Te) as abscissa.

Such media have the problems that when the recording linear velocity isincreased, the preservation stability and the reading optical stabilitydeteriorate, and that when recording density is increased by reducingthe size of the beam spot on the side of the drive, a sufficientsensitivity for use in practice cannot be obtained.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide anoptical recording medium from which the conventional problems in theprior art are eliminated, which has higher correspondence to therecording linear velocity in comparison with that of the conventionaloptical recording media, and excellent preservation stability andreading optical stability, and can be sufficiently used as DVD media.

A second object of the present invention is to provide a recordingmaterial for use in the above-mentioned optical recording medium of thepresent invention.

A third object of the present invention is to provide a method ofproducing the above-mentioned optical recording medium of the presentinvention.

A fourth object of the present invention is to provide a method ofwriting information, reading written information, and rewriting writteninformation in the above-mentioned optical recording medium of thepresent invention.

The first object of the present invention can be achieved by an opticalrecording medium comprising a recording layer which comprises aphase-change recording material which is capable of performing areversible phase change from a crystalline phase to an amorphous phaseand vice versa by light irradiation of the phase-change recordingmaterial,

-   -   the recording layer being capable of writing information        therein, reading written information therefrom, and rewriting        written information by utilizing the reversible phase change of        the phase-change recording material,    -   wherein when a recording mark formed in the recording layer is        repeatedly read 5000 times, using a continuous wave laser beam        having such an intensity Pr that satisfies the condition of        1.1≦R=2.0, in which R is the ratio of a 5000th repeated reading        jitter σ_(repeat) of a recording mark to a 1st reading jitter σ₁        of the recording mark, namely R is σ_(repeat)/σ₁, and a jitter        increasing ratio of the 5000th repeated reading jitter        σ_(repeat) to the 1st reading jitter σ₁ is d (=σ_(repeat)/σ₁),        the optical recording medium satisfies a relationship of        d₁>d_(ow), in which d₁ is the jitter increasing ratio of an        initial writing mark, and d_(ow) is a jitter increasing ratio of        a 1000-times rewritten recording mark.

The inventors have researched into relationship between d₁ which is thejitter increasing ratio of an initial writing mark, and d_(ow) which isa jitter increasing ratio of a 1000-times rewritten recording mark inconventional optical recording media, when a recording mark formed in arecording layer of a conventional optical recording medium is repeatedlyread 5000 times, using a continuous wave laser beam having such anintensity Pr that satisfies the condition of 1.1≦R≦2.0, in which R isthe ratio of a 5000th repeated reading jitter σ_(repeat) of therecording mark to a 1st reading jitter σ₁ of the recording mark, namelyR is σ_(repeat)/σ₁, and a jitter increasing ratio of the 5000th repeatedreading jitter σ_(repeat) to the 1st reading jitter σ₁ is d(=σ_(repeat)/σ₁). As a result, it was discovered and confirmed that inthe conventional optical recording media, d_(ow)>d₁, and this causes theshortcoming of the conventional optical recording media that when therecording linear velocity is increased, the preservation stability andthe reading optical stability deteriorate.

The inventors of the present invention have discovered a recordingmaterial for use in the optical recording medium that makes d_(ow)smaller than d₁, that is, d₁>d_(ow), in the optical recording medium asa result of their studies on the elements to be added to the recordinglayer, the fundamental structure of the phase of the recording material,the composition of the recording material, and the method of producingthe recording layer.

The above-mentioned optical recording medium of the present inventionhas high correspondence to the recording linear velocity, and excellentreading optical stability and preservation stability.

Furthermore, since the recording material in the recording layer is aphase-change recording material which is capable of performing areversible phase change from a crystalline phase to an amorphous phaseand vice versa by light irradiation of the phase-change recordingmaterial, so that writing information in the recording layer, readingwritten information from the recording layer, and rewriting writteninformation in the recording layer can be reversibly performed.

In the above-mentioned optical recording medium, it is preferable thatthe phase-change recording material comprise a pseudo binarycomposition, which has an NaCl-type crystal structure in the crystallinephase, wherein the pseudo binary composition is represented by Sb—TeM,comprising two portions, one portion being represented by Sb, and theother portion being represented by TeM, in which M represents a metalcompound comprising at least Sb or Ge, provided that when TeM is Sb₂Te₃,the pseudo binary composition is a pseudo binary eutectic composition,and that at least one of the portions represented by TeM comprises Sb,and at least one of the other portions represented by TeM comprises Ge.

Furthermore, in the above optical recording medium, it is preferablethat the total content ratio A of Sb and Te in terms of atomic ratiocontained in the pseudo binary composition of the phase-change recordingmaterial be in a range of 0.80≦A≦0.97.

In the above optical recording medium, it is also preferable that thecontents of Sb and Te in terms of atomic ratio contained in the pseudobinary eutectic composition of the phase-change recording materialsatisfy a relationship of 0.65≦Sb/(Sb+Te)≦0.85.

In the above optical recording medium, it is also preferable that thecontent ratio B of Ge in terms of atomic ratio contained in the pseudobinary composition of the phase-change recording material be in a rangeof 0.01≦B≦0.07.

In the above optical recording medium, it is also preferable that thepseudo binary composition of the phase-change recording material furthercomprise as an additional element at least one element selected from thegroup consisting of Ag, In and Bi.

In the above optical recording medium, it is also preferable that thepseudo binary composition of the phase-change recording materialcomprise Sb, Te, Ge, Ag, and In in the respective ranges of atomic ratioof:

-   -   Sb: 0.60 to 0.80,    -   Te: 0.15 to 0.30,    -   Ge: 0.01 to 0.07,    -   Ag: 0.001 to 0.03, and    -   In: 0.02 to 0.09.

The second object of the present invention can be achieved by the samephase-change recording material as described above.

The third object of the present invention can be achieved by a method ofproducing the above-mentioned optical recording medium, comprising thestep of forming the recording layer by performing sputtering, using atarget, with a sputtering power of 0.1 kW to 1.5 kW, the target beingprepared by fusing and mixing a composition composed of a plurality ofelements with a predetermined composition, crushing the composition toprepare a pulverized composition, and sintering said pulverizedcomposition.

The third object of the present invention can also be achieved by amethod of producing the above-mentioned optical recording medium,comprising the step of forming the recording layer by performingsputtering, using the same target as mentioned above, with a sputteringpressure of 0.8 mTorr to 9 mTorr.

The third object of the present invention can also be achieved by amethod of producing the above-mentioned optical recording medium,comprising the step of forming the recording layer by performingsputtering, using the same target as mentioned above, in a sputteringchamber with the pressure in the sputtering chamber being set at avacuum degree of 9×10⁻⁷ Torr or less immediately before the recordinglayer is formed.

The fourth object of the present invention can be achieved by a methodof writing information, reading written information, and rewritingwritten information in the above-mentioned optical recording medium withirradiating the optical recording medium with a laser beam with a spotdiameter of 0.05 μm to 2.0 μm.

The fourth object of the present invention can also be achieved by amethod of writing information, reading written information, andrewriting written information in the above-mentioned optical recordingmedium with irradiating the optical recording medium with a linearrecording speed of 1.2 m/s to 25 m/s, preferably with a linear recordingspeed of 3.5 m/s to 18 m/s.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram showing the relationship between the atomic ratio ofSb/(Sb+Te) and the recording linear velocity and preservation stability.

FIG. 2 is a diagram showing the relationship between the number ofreadings and the light intensity for reading and reading jittercharacteristics.

FIG. 3 shows the results of an analysis by XRD of the crystal structureof each of recording materials at the initial crystallization thereof,which is used in the recording layer of the optical recording medium ofthe present invention.

FIGS. 4A and 4B are comparative diagrams indicating that the addition ofGe to the recording material significantly improves the preservationstability of the optical recording medium.

FIGS. 5 to 8 are graphs indicating the bonding state of the recordingmaterials comprising Sb and Te when additional elements are addedthereto, analyzed by an XAFS analysis, using synchrotron radiation.

FIG. 9 is a graph showing the relationship between the number ofreadings and the jitter characteristics of an optical recording disk inExample 1.

FIG. 10 is a graph showing the relationship between the number ofreadings and the jitter characteristics of an optical recording disk inExample 2.

FIG. 11 is a graph showing the relationship between the number ofreadings and the jitter characteristics of an optical recording disk inExample 3.

FIG. 12 is a graph showing the relationship between the number ofreadings and the jitter characteristics of an optical recording disk inExample 4.

FIG. 13 is a graph showing the relationship between the number ofreadings and the jitter characteristics of an optical recording disk inExample 5.

FIG. 14 is a graph showing the relationship between the number ofreadings and the jitter characteristics of an optical recording disk inComparative Example 1.

FIG. 15 is a graph showing the relationship between the number ofreadings and the jitter characteristics of an optical recording disk inComparative Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The optical recording medium of the present invention comprises arecording layer comprising a phase-change recording material which iscapable of performing a reversible phase change from a crystalline phaseto an amorphous phase and vice versa by light irradiation of thephase-change recording material,

-   -   the recording layer being capable of writing information        therein, reading written information therefrom, and rewriting        written information by utilizing the reversible phase change of        the phase-change recording material,    -   wherein when a recording mark formed in the recording layer is        repeatedly read 5000 times, using a continuous wave laser beam        having such an intensity Pr that satisfies the condition of        1.1≦R≦2.0, in which R is the ratio of a 5000th repeated reading        jitter σ_(repeat) of a recording mark to a 1st reading jitter σ₁        of the recording mark, namely R is σ_(repeat)/σ₁, and a jitter        increasing ratio of the 5000th repeated reading jitter        σ_(repeat) to the 1st reading jitter σ₁ is d (=σ_(repeat)/σ₁),        the optical recording medium satisfies a relationship of        d₁>d_(ow), in which d₁ is the jitter increasing ratio of an        initial writing mark, and d_(ow) is a jitter increasing ratio of        a 1000-times rewritten recording mark.

In the mark length modulation system in the present invention, the term“jitter” is defined by a value of σ/Tw, wherein σ is a standarddeviation of the leading and trailing edges of a recording mark withreference to clock signals, and Tw is a detection window width of theclock signals.

FIG. 2 shows the relationship between the reading light intensity andreading jitter characteristics and the number of readings. In thefigure, σr denotes the reading jitter from the 2nd reading on.

In the above-mentioned optical recording medium of the presentinvention, the continuous wave laser beam having such an intensity Prthat satisfies the condition of 1.1≦R≦2.0, wherein R=σ_(repeat)/σ₁, hasan intensity which is about 1.1 to 2 times greater than that of acontinuous wave laser beam used in the conventional drives. The use ofthis continuous wave laser beam is intended to subject the recordingmark to forced and accelerated deterioration. In other words, in theconventional drives, for instance, when a laser beam is used forirradiation, the irradiation is carried out with a power of 0.3 mW to0.8 mW, while in the measurement for the present invention, theirradiation is carried out with a power of 0.7 mW to 1.5 mW. In thiscase, the recording mark is formed with a conventional recording powerof about 8 mW to 20 mW. The recording linear velocity is in the range ofabout 3.5 m/s to about 8.5 m/s. The light spot diameter is about 0.8 μmto 1.0 μm.

Studies made by the inventors of the present invention indicated thatthe optical recording media, in which d_(ow)>d₁, have the shortcomingthat when the recording linear velocity is increased, the preservationstability and the reading optical stability deteriorate, and in contrastto this, when the optical recording media in which a recording materialwith d_(ow)<d₁ is used exhibit excellent correspondence to recordinglinear velocity, reading optical stability and preservation stability.

As mentioned above, the optical recording medium of the presentinvention satisfies the relationship of d₁>d_(ow), in which d₁ is thejitter increasing ratio of an initial writing mark, and d_(ow) is ajitter increasing ratio of a 1000-times rewritten recording mark, whenreading is performed under the above-mentioned conditions.

The optical recording medium which satisfies the above-mentionedconditions has excellent correspondence to recording linear velocity,reading optical stability and preservation stability and can besufficiently used for DVD-RW.

As mentioned above, the recording layer of the optical recording mediumof the present invention comprises a phase-change recording materialwhich is capable of performing a reversible phase change from acrystalline phase to an amorphous phase and vice versa by lightirradiation of the phase-change recording material.

The phase-change recording material may comprise a pseudo binarycomposition, which has an NaCl-type crystal structure in the crystallinephase, wherein the pseudo binary composition is represented by Sb—TeM,comprising two portions, one portion being represented by Sb, and theother portion being represented by TeM, in which M represents a metalcompound comprising at least Sb or Ge, provided that when TeM is Sb₂Te₃,the pseudo binary composition is a pseudo binary eutectic composition,and that at least one of the portions represented by TeM comprises Sb,and at least one of the other portions represented by TeM comprises Ge.

The above-mentioned pseudo binary composition and the NaCl-type crystalstructure in the crystalline phase can be identified by an XRD (X-raydiffraction) analysis and an XAFS (Extended X-ray Absorption FineStructure) analysis.

FIG. 3 shows the data of the crystalline structures of recordingmaterials, SbTe, AgInSbTe, and AgGeInSbTe, at the initialcrystallization thereof for use in the recording layer of the opticalrecording medium of the present invention, obtained by XRD. The dataindicated that each of these materials can be indexed as having theNaCl-type crystal structure.

In the above optical recording medium, it is preferable that the totalcontent ratio A of Sb and Te in terms of atomic ratio contained in thepseudo binary composition of the phase-change recording material be in arange of 0.80≦A≦0.97 from the viewpoints of the recording linearvelocity and the reading optical stability.

In the above optical recording medium, it is also preferable that thecontents of Sb and Te in terms of atomic ratio contained in the pseudobinary eutectic composition of the phase-change recording materialsatisfy a relationship of 0.65≦Sb/(Sb+Te)≦0.85 from the same viewpointsas mentioned above. The tendency shown in FIG. 1 is considered tosupport this.

In the above optical recording medium, it is also preferable that thecontent ratio B of Ge in terms of atomic ratio contained in the pseudobinary composition of the phase-change recording material be in a rangeof 0.01≦B≦0.07, more preferably 0.01≦B≦0.05. The thus added Ge serves tohinder the reduction of the preservation stability when thecorrespondence to the recording linear velocity is further improved inthe above-mentioned relationship of the composition of Sb and Te.

FIG. 4A and FIG. 4B respectively compare the jitter characteristics whenGe was not added, and the jitter characteristics when Ge was added,namely FIG. 4A is for Ag_(2.5)In_(3.0)xSb_(71.5)Te_(23.0), and FIG. 4Bis for Ag_(0.5)In_(3.0)Sb_(71.5)Te_(23.0)Ge_(2.0).

For the comparison of the jitter characteristics, two sample opticalrecording media were fabricated by sputtering on a disk-shaped substratemade of polycarbonate with a diameter of 120 mm and a thickness of 0.6mm the following layered structure composed of (a) an underocat layercomposed of Zn.SiO₂ with a thickness of 100 nm, (b) a recording layercomposed of any of the above-mentioned recording materials(Ag_(2.5)In_(3.0)xSb_(71.5)Te_(23.0) orAg_(0.5)In_(3.0)Sb_(71.5)Te_(23.0)Ge_(2.0)) with a thickness of 20 nmformed on the first protective layer, (c) a protective layer composed ofZn.SiO₂ with a thickness of 20 nm formed on the recording layer, and (d)a reflection layer made of Al with a thickness of 140 nm formed on theprotective layer.

The thus fabricated two optical recording media were preserved at 80°C., 85% RH, and the jitter characteristics thereof were then measured bywriting a recording mark and rewriting the recording mark.

The results of the measurement of the jitter characteristics are shownin FIG. 4A and FIG. 4B. In the graphs in FIG. 4A and FIG. 4B, the numberfor each curve denotes the number of rewriting, provided that the number1 indicates writing of the recording mark one time.

The results of the measurement of the jitter characteristics shown inFIG. 4A and FIG. 4B indicate that there is a tendency that thepreservation stability is significantly improved by the addition of Geto the recording material.

It is preferable that a recording material comprising Sb, Te and Ge foruse in the recording layer have the following composition of Sb, Te andGe in terms of atomic ratio thereof:

-   -   Sb: 0.60 to 0.80, more preferably 0.63 to 0.75, further more        preferably 0.65 to 0.75,    -   Te: 0.15 to 0.30, more preferably 0.20 to 0.25,    -   Ge: 0.01 to 0.07, more preferably 0.02 to 0.05.

When the content of Sb is smaller than the above-mentioned broadestrange, an optical recording disk with high recording liner velocitytends to become difficult to obtain, while when the content of Sb isgreater than the above-mentioned broadest range, an optical recordingdisk with high preservation stability tends to become difficult toobtain.

When the content of Te is smaller than the above-mentioned broadestrange, the preservation stability and the rewriting characteristics tendto deteriorate, while when the content of Te is greater than theabove-mentioned broadest range, the optical recording disk tends tobecome unsuitable for recording with high linear velocity.

When the content of Ge is smaller than the above-mentioned broadestrange, the preservation stability tends to deteriorate, while when thecontent of Ge is greater than the above-mentioned broadest range, theoptical recording disk tends to become unsuitable for recording withhigh linear velocity.

In the optical recording medium of the present invention, the pseudobinary composition of the phase-change recording material may furthercomprise as an additional element at least one element selected from thegroup consisting of Ag, In and Bi, in order to improve thecharacteristics of the optical recording medium such as the modulationdegree of the recording marks. When the above-mentioned additionalelement is added to the phase-change recording material, it ispreferable that the atomic ratio thereof be in the range of about 0.01to 0.09.

FIGS. 5 to 8 show the results of the analysis of recording materialsincluding the above-mentioned additional elements with respect to thebonding state thereof by an Sb—K-Edge XAFS analysis using synchrotronradiation. The results of the analysis indicate that the additionalelements are bonded to Te.

In the above-mentioned optical recording medium of the presentinvention, it is preferable that the pseudo binary composition of thephase-change recording material comprise Sb, Te, Ge, Ag, and In in therespective ranges of atomic ratio of:

-   -   Sb: 0.60 to 0.80, more preferably 0.63 to 0.75, furthermore        preferably 0.65 to 0.75,    -   Te: 0.15 to 0.30, more preferably 0.20 to 0.25,    -   Ge: 0.01 to 0.07, more preferably 0.02 to 0.05,    -   Ag: 0.001 to 0.03, more preferably 0.001 to 0.02, and    -   In: 0.02 to 0.09, more preferably 0.03 to 0.09.

When the content of Sb is smaller than the above-mentioned broadestrange, the optical recording disk tends to become unsuitable forrecording with high recording liner velocity, while when the content ofSb is greater than the above-mentioned broadest range, the preservationstability thereof tends to deteriorate.

When the content of Te is smaller than the above-mentioned broadestrange, the preservation stability tends to deteriorate, while when thecontent of Te is greater than the above-mentioned broadest range,recording with high recording linear velocity tends to become difficultto perform, and the rewriting characteristics tend to deteriorate.

When the content of Ge is smaller than the above-mentioned broadestrange, the preservation stability tends to deteriorate, while when thecontent of Ge is greater than the above-mentioned broadest range, theoptical recording disk tends to become unsuitable for recording withhigh recording linear velocity.

When the content of Ag is smaller than the above-mentioned broadestrange, the jitter characteristics and the modulation degree tend todeteriorate, while when the content of Ag is greater than theabove-mentioned broadest range, recording with high recording linearvelocity tends to become difficult to perform.

When the content of In is smaller than the above-mentioned broadestrange, the modulation degree tends to become small, while when thecontent of In is greater than the above-mentioned broadest range, thereading optical stability of the optical recording medium tends todeteriorate.

The optical recording media using the phase-change recording materialswith the above-mentioned compositions exhibit extremely excellent C/Nratio, erasing ratio, sensitivity, jitter characteristics,correspondence to recording linear velocity, reading optical stability,and preservation stability, and accordingly can be sufficiently used inDVD-RW.

The recording layer of the optical recording medium of the presentinvention can be formed by a conventional film formation method such assputtering. When the recording layer was formed by sputtering in thepresent invention, the sputtering was conducted by DC magnetronsputtering under the conditions that the applied power was 0.5 kW, thepressure of Ar gas was 2 mTorr, and the back pressure in a filmformation chamber immediately before the formation of the recordinglayer was 1×10⁻⁷ Torr.

Under the above-mentioned conditions for the sputtering, the sputteringrate for the formation of the recording layer was 1.2 times or more thanthe sputtering rate in the Rf sputtering which was adopted previously.Thus, in comparison with the Rf sputtering, the DC magnetron sputteringis capable of forming the recording layer in a shorter time.Furthermore, the DC magnetron sputtering is capable of forming therecording layer comprising the recording material with theabove-mentioned crystal structure, composition, and characteristics.

It is preferable that the recording layer have a thickness of 5 nm to 30nm, more preferably a thickness of 10 nm to 20 nm.

The optical recording medium of the present invention comprises theabove-mentioned recording layer which is provided on a substrate.

As the materials for the substrate, glass, ceramics, and resins can beemployed. Of these materials, resins are preferable for forming thesubstrate. This is because a resin substrate is advantageous over othersubstrates in terms of fabrication and cost. Representative examples ofthe resin for forming the substrate are polycarbonate resin, epoxyresin, polystyrene resin, acrylonitrile-styrene copolymer resin,polyethylene resin, and polymethyl methacrylate resin. In view of theworkability and optical characteristics, polycarbonate resin is the mostpreferable of the above-mentioned resins. When a substrate made ofpolycarbonate is used, it is preferable to provide an ultravioletabsorbing layer on a light-incident side of the substrate.

The substrate can be variously shaped, for example, disk-shaped,card-shaped, or sheet-shaped, with an arbitrary thickness, such as 1.2mm, 0.6 mm, and 0.3 mm.

In the optical recording medium of the present invention, whennecessary, there can be provided an undercoat layer between thesubstrate and the recording layer, a protective layer on the recordinglayer, and a hard coat layer on the protective layer.

Furthermore, there can be provided a reflection layer between theprotective layer and the hard coat layer.

Furthermore, it is preferable that an ultraviolet absorbing layer beprovided on at least one side of the optical recording medium.

As the materials for the undercoat layer and the protective layer, therecan be employed oxides such as SiO, SiO₂, ZnO, SnO₂, Al₂O₃, TiO₂, In₂O₃,MgO and ZrO₂, nitrides such as Si₃N₄, AlN, TiN, BN and ZrN, sulfidessuch as ZnS, In₂S₃ and TaS₄, carbides such as SiC, TaC, B₄C, WC, TiC andZrC, diamond-like carbon, and mixtures thereof.

The undercoat layer and the protective layer can be formed, forinstance, by sputtering, ion-plating, vacuum deposition, and plasma CVD.

It is preferable that the undercoat layer have a thickness of 20 nm to250 nm, more preferably 40 nm to 250 nm, furthermore preferably 160 nmto 250 nm.

It is preferable that the protective layer have a thickness of 5 nm to150 nm, more preferably 10 nm to 30 nm.

As the materials for the reflection layer, there can be employed metalmaterials such as Al, Ag and Au, and the metal materials to whichadditive materials such as Ti, Cr, and Si are added.

The reflection layer can be formed, for instance, by sputtering,ion-plating, vacuum deposition, and plasma CVD.

It is preferable that the reflection layer have a thickness of 50 nm to200 nm, more preferably 80 nm to 150 nm.

It is preferable that the materials for the hard coat layer beultraviolet curing resins such as urethane acrylate, acrylate, andmixtures thereof, although the materials therefor are not limited tosuch ultraviolet curing resins.

It is preferable that the hard coat layer have a thickness of about 1 μmto about 50 μm.

When information is written in the optical recording medium of thepresent invention, and written information is read therefrom, as thelight for writing and reading, there can be employed various kinds oflight beams other than laser beam. It is preferable that the spotdiameter of the beams be in the range of 0.05 μm to 2 μm, morepreferably in the range of 0.1 μm to 1.6 μm.

It is preferable that the power of the writing light be about 8 mW toabout 20 mW, and the power of the reading light be about 0.3 mW to about1.0 mW.

It is preferable that the writing light and the reading light have awavelength of 200 nm to 1000 nm, more preferably a wavelength of 400 nmto 800 nm.

It is preferable that the recording linear velocity be in the range of 1m/s to 30 m/s, more preferably in the range of 1.2 m/s to 18 m/s.

Other features of this invention will become apparent in the course ofthe following description of exemplary embodiments, which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLE 1

On a polycarbonate disk-shaped substrate with a groove having a trackpitch of 0.74 μm and a depth of about 500 Å formed thereon, a recordinglayer was formed by use of a target with the composition ofSb_(74.0)Te_(21.0)Ge_(5.0) as shown in TABLE 1 by DC magnetronsputtering under the following conditions: Back pressure: 1 × 10⁻⁷ TorrGas used: Ar Applied power: 0.5 kW Gas pressure: 2 mTorr

On the thus formed recording layer, a reflection layer made of AlTi witha thickness of 140 nm was formed, whereby an optical recording disk No.1 of the present invention was fabricated.

EXAMPLES 2 TO 5 AND COMPARATIVE EXAMPLES 1 AND 2

The procedure of the fabrication of the optical recording disk No. 1 ofthe present invention in Example 1 was repeated in the same manner as inExample 1 except that the target used in Example 1 was replaced by therespective targets with the compositions as shown in TABLE 1, wherebyoptical recording disks No. 2 to No. 5 of the present invention, andcomparative optical recording disks No. 1 and No. 2 were fabricated.TABLE 1 Composition of Target Ex. 1 Sb_(74.0)Te_(21.0)Ge_(5.0) Ex. 2Sb_(72.2)Te_(23.8)Ag_(0.5)In_(3.0)Ge_(0.5) Ex. 3Sb_(71.5)Te_(24.0)Ag_(0.5)In_(3.0)Ge_(1.0) EX. 4Sb_(71.5)Te_(23.0)Ag_(0.5)In_(3.0)Ge_(2.0) Ex. 5Sb_(68.5)Te_(24.0)Ag_(0.5)In_(5.0)Ge_(2.0) Comp. Sb_(75.0)Te_(25.0) Ex.1 Comp. Sb_(71.5)Te_(23.0)Ag_(2.5)In_(3.0) Ex. 2

Each of the thus fabricated optical recording disks No. 1 to No. 5 ofthe present invention, and the comparative optical recording disks No. 1and No. 2 were evaluated with respect to the preservation stability andthe reading optical stability, and the comparison of the magnitudes ofd₁ and d_(ow) under the following conditions, provided that all of thetested optical recording media had a good sensitivity with a modulationdegree of 50% or more when recording marks were written by a laser beamwith a predetermined power: R (=σ_(repeat)/σ₁):  1.2 to 1.5 Wavelengthof laser beam:  660 nm Spot diameter of laser beam:  0.9 μm NA ofobjective: 0.65 Reading linear velocity: DVD ROM equal velocity (3.5m/s)

Each of the above-mentioned characteristics was evaluated as follows:

[Preservation Stability]

Each optical recording disk with a recording mark recorded therein waspreserved at a temperature as high as 80° C. to 90° C. and a humidity ashigh as 85% RH, and the recording characteristics (jittercharacteristics) were compared before and after the preservation.

[Reading Optical Stability]

Each optical recording disk was continuously irradiated with a readinglight beam with a power of about 0.5 mW to about 1.5 mW, and the changesin the jitter characteristics in the reading were monitored.

[Comparison of the Magnitudes of d₁ and d_(ow)]

The magnitudes of d₁ and d_(ow) which are respectively defined in theabove, were compared in terms of the jitter characteristics at the5000^(th) of the repeated readings, and with and without rewritings.

The results of the above evaluation are shown in TABLE 2 and also inFIG. 9 to FIG. 15. TABLE 2 Reading Preservation optical d₁ − d_(ow)stability stability Example 1 + ◯ ◯ Example 2 + ◯ ◯ Example 3 + ◯ ◯Example 4 + ◯ ◯ Example 5 + ◯ ◯ Comp. Ex. 1 − X ◯ Comp. Ex. 2 − X ◯

In the above TABLE 2, the symbol “◯” denotes that the characteristicsare suitable for use in practice, and the symbol “X” denotes that thecharacteristics are not suitable for use in practice.

The results shown in the above TABLE 2 indicate that the opticalrecording disks of the present invention have excellent preservationstability and reading optical stability. Furthermore, the opticalrecording disks of the present invention had a correspondence torecording linear velocity of 8 m/s to 13.5 m/s, which was excellent.This corresponds to 2 to 3 times the recording linear velocity ofDVD-ROM in the same recording density as that of DVD-ROM.

Japanese Patent Application No. 2000-099757, filed Mar. 31, 2000, ishereby incorporated by reference.

1-49. (canceled)
 50. An optical recording medium comprising a recordinglayer which comprises a phase-change recording material which is capableof performing a reversible phase change from a crystalline phase to anamorphous phase and vice versa by light irradiation of said phase-changerecording material, said recording layer being capable of writinginformation therein, reading written information therefrom, andrewriting written information by utilizing said reversible phase changeof said phase-change recording material, wherein when a recording markformed in said recording layer is repeatedly read 5000 times, using acontinuous wave laser beam having such an intensity Pr that satisfiesthe condition of 1.1≦R≦2.0, in which R is the ratio of a 5000^(th)repeated reading jitter σ_(repeat) of a recording mark to a 1^(st)reading jitter σ₁ of said recording mark, namely R is σ_(repeat)/σ₁, anda jitter increasing ratio of said 5000^(th) repeated reading jitterσ_(repeat) to said 1^(st) reading jitter σ₁ is d (=σ_(repeat)/σ₁), saidoptical recording medium satisfies a relationship of d₁>d_(ow), in whichd₁ is said jitter increasing ratio of an initial writing mark, andd_(ow) is a jitter increasing ratio of a 1000-times rewritten recordingmark, wherein said phase-change recording material comprises a pseudobinary composition which has an X-ray diffraction spectrum such that afirst peak is observed at a Bragg (2θ) angle of 27° to 29° and a secondpeak is observed at a Bragg (2θ) angle of 40° to 43° when a CuKα linehaving a wavelength of 1.54 Å is used, wherein the second peak is widerthan the first peak, wherein said pseudo binary composition isrepresented by Sb—TeM, comprising two portions, one portion beingrepresented by Sb, and the other portion being represented by TeM, inwhich M represents a metal compound comprising at least Sb or Ge,provided that when TeM is Sb₂Te₃, said pseudo binary composition is apseudo binary eutectic composition, and that at least one of theportions represented by TeM comprises Sb, and at least one of the otherportions represented by TeM comprises Ge.
 51. The optical recordingmedium as claimed in claim 50, wherein said pseudo binary compositionhas an X-ray diffraction spectrum that further includes a third peakobserved at a Bragg (2θ) angle of 51° to 52.5°, and a fourth peakobserved at a Bragg (2θ) angle of 58.5° to 61°.